The invention relates to an endoscope lens in accordance with the general concept of patent claim 1, and to an endoscope with a lens of the same type.
Conventional endoscopes have a lens arranged distally which projects an image of the scene to be observed, and an image transmitter which conveys the image projected by the lens from the distal to the proximal end of the endoscope, where it is observed, for instance, by an eyepiece or recorded by a video camera. The image transmitter in these devices generally consists of lens systems, the relay lens systems which form the image in each case from one successive image plane to the next. Endoscope lenses must achieve a very great angle of sight (2w up to 80 degrees), and all lenses must have an equally small diameter so that they can be accommodated within a narrow tube.
Lenses used in known endoscopes consist primarily of two lenses or lens groups which are separated by the aperture diaphragm; here and in the remainder of this text, xe2x80x9caperture diaphragmxe2x80x9d shall be understood as also including the image of the aperture diaphragm, since additional reproducing systems are usually added. The first lens or lens group in the beam direction, mounted ahead of the aperture diaphragm, has negative total refractive power in order to achieve the greatest possible image field angle, and usually consists of a single negative lens. Beyond the aperture diaphragm, a second lens group is installed whose total refractive power is positive and which normally consists of several lenses. The aperture diaphragm between the two lens groups is placed in the front focal point of the second lens group, so that the principal beams run on the image side parallel to the optical axis; this is favorable for coupling onto the relay lens systems. The foregoing state of the art is referred to in FIGS. 1 to 3c of U.S. Pat. No. 5,005,957 or the article xe2x80x9cTelecentric relay systems having distortion and their application in endoscopes,xe2x80x9d by Eberhard Dietzsch, SPIE, Vol. 2774 (pp. 276-282).
A disadvantage of the known endoscope lenses described here is the strong, negative, that is, barrel-shaped, distortion that depends on the visual field angle, a distortion that for example reaches values of xe2x88x9220% . . . 25% at a visual field angle of 2w=70 degrees . . . 80 degrees. It has long been believedxe2x80x94as noted in the second paragraph of the above-cited articlexe2x80x94that this distortion of endoscope lenses cannot be reduced, at least, if the lens includes lenses exclusively of the standard technology, that is, with spherical surfaces and constant refractive index.
U.S. Pat. No. 5,005,957 therefore suggests that lenses with aspherical surfaces be used. In this way it is possible to reduce the distortion to a few percentage points. Because aspherical surfaces in many cases can be replaced by lenses of a material with axially varying refractive index, it has also been suggested that the distortion of the known endoscope lenses should be corrected by the use of lenses whose refractive index varies in the axial direction. Both the use of lenses with aspherical surfaces and the use of lenses with varying refractive index result in higher manufacturing costs in comparison with conventional lenses with spherical surfaces and materials of constant refractive index, as well as because of production tolerances of additional image errors on the image periphery.
As the aforementioned article by Eberhard Dietzsch has also shown, it is possible to use asymmetrical relay lens systems whose distortion is opposite to the distortion of the lens, and thus partially corrects it. This solution, however, has the disadvantage that the correction depends on the number of relay lens systems in use and thus on the length of the individual endoscope. Asymmetrical relay lens systems, moreover, are more expensive to produce than the symmetrical ones. Asymmetry, in addition, introduces mainly more image errors such as coma and lateral chromatic error, which are sometimes difficult to correct.
Another approach is taken by JP 60-80816A, according to which at least one positive lens and at least one negative meniscus lens are called for in the first lens group in the beam direction, in order to compensate for the distortion. In accordance with EPO 571 725 A1, the first lens group consists of two meniscus lenses, each having negative refractive power, which in a separate version are cemented together. Lenses that contain meniscus lenses, particularly in the first lens group, have the disadvantage, however, that they are more difficult to center, since the centers of curvature of the two lens surfaces are on the same side of the lens, especially when the two centers of curvature are close together. This is even more true if two or more meniscus lenses are arranged in a row.
Thus the invention is based on the task of providing an endoscope lens which, even at great angles of sight as high as 80 degrees, only have a distortion that cannot be detected by the human eye, whereby the elements of the lens and the endoscope lens itself should be economical to produce and to assemble.
The invention is based on an endoscope lens with a first lens group in the beam direction having a negative total refractive power, an aperture diaphragm that is installed beyond the first lens group, and a second lens group whose total refractive power is positive and which is assembled beyond the aperture diaphragm. To avoid the expense associated with aspherical lens surfaces or non-homogeneous materials, and the related reproduction errors, only spherical surfaces and only materials with homogeneous refractive index are used.
The contribution of one lens to the total distortion of the lens system is determined by the size and algebraic sign of the refractive power of the lens and by the position of the lens relative to the aperture diaphragm. To compensate for the distortion produced by the known endoscope lens mentioned at the outset, the invention""s endoscope lens therefore contains, in the first lens group, which has negative total refractive power, at least one lens with positive refractive power. Here, a positive lens is first installed in the beam direction and at this lens the refractive power and distance from the aperture diaphragm are measured in such a way that the distortion produced by this lens appreciably corrects the negative distortion that the lens system would otherwise have. Since the total refractive power of the first lens group is negative, the first lens group in addition must contain at least one negative lens, whereby the sum of the contributions of the refractive powers of the negative lenses in the first lens group must be greater than the sum of the contributions of the refractive powers of the positive lenses in the first lens group.
The above-mentioned task is fulfilled, therefore, by the invention because the negative lenses in the first lens group are biconcave or plano-concave lenses.
Such lenses are simpler to produce, simpler to center, and simpler to install in the total optical system than the negative meniscus lenses used in known endoscope lens systems. Thus, contrary to the viewpoint cited above and without the costs associated with the known endoscope lens systems, it is possible to construct endoscope lenses of the mentioned type in standard technology in such a way that the distortion of the lens can be reduced to a level that is not perceived as disturbing for the viewer. In particular, the absolute value of the total distortion can thus be reduced to a value of less than 6%, typically even 4% or less.
The determination of the parameters of the lenses, and the adjustment of the other elements of the lens system, proceed, on the basis of the aforementioned specifications, in known manner, with the use of customary optic design software. Surprisingly, it is possible with the invention""s solution, not only to reduce the distortion to non-disturbing levels, but also to continue to correct the other image errors as well, and for instance even to improve the solution further. The image field barreling of the lens system can be selected, in known manner, in such a way that it compensates for any image field barreling of the relay lens systems, or the image field of the lens system may also be level.
In an advantageous realization of the invention, the first lens group consists only of two lenses, one of which has positive and the other negative refractive power, whereby the positive lens is installed in the beam direction ahead of the negative lens. In this realization, the lens system is especially simple and economical.
In another advantageous realization of the invention, the positive lens in the first lens group is a biconvex lens, in which the more strongly curved surface is preferably on the image side, or is a plano-convex lens whose convex surface is preferably installed on the image side. As a result, a good correction of the reproduction error is achieved.
In another advantageous realization, the positive lens in the first lens group is cemented with the negative lens. This results in a cemented element, which is particularly simple to maneuver and, contrary to individual meniscus lenses, can also be centered without problems. In this case it is particularly advantageous if the refractive index of the positive lens is greater than that of the negative lens, contrary to the customary proportion with cemented elements. As a result, a particularly good correction of the reproduction error is achieved.
The positive lens may also have an empty space separating it from the negative lens. The space constitutes an additional parameter in the design of the lens system, one that can be used for improving the correction.
It is another task of the present invention to produce an endoscope in which, despite a wide angle of sight, the distortion is so minor that it cannot be perceived by the human eye, whereby the optical elements of the endoscope should be simple to produce and to install.
The invention fulfills this task in that a largely distortion-free eyepiece, an equally largely distortion-free image transmitter, and a lens system are installed in an endoscope in accordance with one of claims 1 to 8.
Distortion-free image transmitters are known, for example symmetrical relay lens systems. Also common are eyepieces that are distortion-free or produce only low levels of distortion; this residual distortion is primarily negative in known eyepieces and thus cannot be compensated by the negative distortion of most of the known endoscope lenses. Because, according to the invention, the three structural components of the optical system of an endoscope are each in their own right, at least to a great extent, corrected to distortion-free status, it is possible to have a correction of the reproduction error that is optimal and valid for various overall lengths and combinations. Because an endoscope lens according to this invention is used, the production expense for the endoscope lens is minor.
The invention""s endoscope lens can also be used to good advantage together with a flexible image transmitter or in an endoscope with a distally mounted electronic image receptor. The term xe2x80x9cendoscopexe2x80x9d is understood here to include also other types of viewing devices such as periscopes.